fix string Null in yaml.
fix string Null in yaml.

dotnet/docfx,pascalberger/docfx,LordZoltan/docfx,dotnet/docfx,superyyrrzz/docfx,LordZoltan/docfx,DuncanmaMSFT/docfx,LordZoltan/docfx,superyyrrzz/docfx,928PJY/docfx,DuncanmaMSFT/docfx,dotnet/docfx,928PJY/docfx,928PJY/docfx,hellosnow/docfx,hellosnow/docfx,pascalberger/docfx,pascalberger/docfx,superyyrrzz/docfx,LordZoltan/docfx,hellosnow/docfx